drm/i915/gvt: use drm->debugfs_root for creating debugfs files

Since commit 0b30d57a ("drm/debugfs: rework debugfs directory
creation v5") we should be using drm->debugfs_root instead of
minor->debugfs_root for creating debugfs files.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/482a3516e00b2885cd62f872ad09f51a9d8176b4.1753782998.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>